Bacteriophage T7 infection of Escherichia coli cells containing F plus or substituted F' PIF plus sex factors is abortive. No T7 late proteins are synthesized from T7 late mRNA shown to be present in these cells. In vitro protein synthesizing systems derived from T7 infected F'lac plus containing cells show deficiencies in the ribosomes and in a soluble factor necessary for protein synthesis. Defective extracts can be restored to activity by the addition of the soluble fraction obtained from uninfected F'lac plus containing cells or from uninfected or T7 infected F- cells. 1) PIF- mutants of the sex-factor, partially or completely defective in the inhibition of T7 development have been isolated. We will determine the number of cistrons involved in the PIF response, study the control of PIF gene expression, and attempt to identify the PIF proteins. 2) Host mutations which allow T7 development in the presence of a PIF plus F'lac sex-factor have been isolated and will be mapped on the E. coli chromosome. We will determine the mechanism of action of these pim mutations and study their interaction with certain rifR RNA polymerase mutations which enhance or suppress the pim effect. 3) We will study the steps leading to the loss of protein synthetic ability and purify the protein factor in the soluble fraction which restores activity.